Tarnish-resistant bronze powder and treatment process therefor



Patented Jan. 19, 1954 TARNISH-RESISTANT BRONZE POWDER AND TREATMENT PROCESS FOR THERE- Joseph Halberstadt, Malone, N. Y., assignor to Malone Bronze Powder Works Inc., Malone,

No Drawing. Application May 13, 1950, Serial No. 161,895

4 Claims.

The present invention relates to a method or process for obtaining sc-called bronze powders of improved qualities and, more particularly, having a greatly increased resistance against discoloration or tarnish.

The appellation bronze powder for the metallic pigments widely used to simulate gold and gilt is, of course incorrect, strictly speaking, because such pigments are primarily constituted of copper and copper alloys, mainly brasses and the like. But, this bronze designation ha been associated with the industry and trade for the past '75 years at least and, accordingly, thi term will be used throughout this test for defining copperbase metallic pigment particles of flake-like structure quite unlike other metallic aggregates of spheroidal or nodular for .1 obtained by chemical, electrolytic means or atomization.

The flake form of bronze powder particles is a function of the disintegration carried out generally on a metallic aggregate in hammer and ball mills; as the beating operation proceeds, in presence of a suitable fatty lubricant, the flattened particles are gradually broken down into smaller and thinner flakes having a thickness of approximately 0.30 from 0.40 to 1.00 micron and an average cross section of from 5 to 150 microns and more, depending on the fineness desired in the final product. So, in any case, an average bronze powder has an average particle breadth to thickness ratio exceeding 60 to 1 and often 100 to 1.

Unfortunately, however, such bronze powders are inherently subject to discoloration by oxidation, attack by chemical agents, fumes acting directly on the base constitutive metals.

This discoloration is known as tarnish and, so far, little progress has been made in the search for economical means and methods of protection against tarnishing. Obviously, this is a problem 01' some magnitude, due to the tremendous surface of the powder compared to a solid metal volume of comparable weight, and the tendency. of the powder to retain oxidation products and occluded gases and moisture initiating further tarnishing action. Consequently, this lack of stability against discoloration has been a great drawback and restricted to a great extent the wider application of bronze powder in the graphic arts, decoration in general and, lately, the plastic industry.

Quoting from the February issu 1949, page 66 of Modern Plastics, as a case in point:

"Aluminum Powder is the metal used since research by Vinyls-resin makers has shown that copper, copper-zinc alloys and other metals can not be used with assurance of good color control.

This is true of all commercial bronze powders known so far which, when incorporated in certain plastics, lose their color due to heat and chei'nical influences, for instance hydrochloric acid devel oped in vinyl-chloride plastics.

The present invention has been conceived to avoid the disadvantages noted above in copperbase bronze powders and its objects may be stated as follows.

The main object or" the invention resides in the provision of an improved bronze powder having wider adaptability in the graphic arts and the plastic industry, its primary characteristic being a greatly increased resistance against the attacks or agents causing tarnish.

Another important object of the invention is the provision of a bronze powder of the character noted immediately above and which is obtainable easily at relatively low cost.

As a further object of great importance also, the invention contemplates the production of a tarnish-resistant copper-base bronze powder having higher reflectivity and retaining the working and. physical characteristics of particle size and thickness inherent to conventional non-tarnishresistant powders.

Still another object of the invention concerns a bronze powder of the character described which is additionally highly resistant to heat and to the oxidation discoloration resulting therefrom.

Other objects and advantages of the invention will be specifically pointed out, or become apparent, in the description to follow.

Briefly stated, the process of the invention is a treatment given to already-produced powders, that is; powders that have been obtained by conventional means in the ordinary manner and of any suitabl particle size and thickness. Conse quently, said process is of a chemical nature, according to which each powder particle is intended to be, and actually is, coated with a protective, impermeable, invisible and microscopically thin substance isolating the metal from its surrounding medium.

Basically, also, the process of the invention is a heat treatment conducted under carefully selected conditions and limited to copper and copperbase alloys; silver, nickel and copper-nickel-zinc alloys can be treated successfully also, although less important commercially than copper and copper-zinc bronze powders.

Aluminum powders cannot be rendered tarnish resistant by means of the present process; as a matter of fact, aluminum powders have been found to be more reactive after than before treatment.

The film, or isolating substance which, according to the invention, coats each particle of a treated bronze powder has not been, so far, fully recognized and. its exact nature is still open to conjecture. Recent studies, however, show that such films obtained under conditions to be described below are complex polymerization prods ucts quite insoluble in organic solvents and contain crystallites of graphitic carbon. In any case, however, in view of the results obtained, 1. e. the increased brilliancy and reflectivity of the treate bronze powder, it is clear that said film must be transparent and, yet, highly impermeable to the fumes and agents causing tarnish.

This film is caused to appear on the bronze powder particles after heating this powder in a given atmosphere having no decarburizing properties, the temperature being somewhat critical and such as to achieve transformation of the inherent lubricant coating on the particles into the complex compound noted above.

This lubricant coating is present in all commercial bronze powders in various amounts: its presence is determined by means of a standard analytical procedure consisting in dissolving the powder in nitric acid, the lubricant remaining as a residue of greasy matter substantially soluble in organic solvents such as ether.

When, according to the invention, the same powder is previously heated to, say, 390 C. in a suitable atmosphere, propane gas for example, a transformation has taken place because the treated powder now yields a different residue when dissolved in the acid: this residue is now only partly soluble in ether, the remainder being blackish and quite insoluble in the organic solvent. Furthermore, the so-heated bronze powder is already resistant to tarnishing agents, such as hydrogen sulphide.

Increasing the temperature to 500 C. has the remarkable effect of greatly improving the resistance and, at the same time, decreasing the soluble residue to the vanishing point almost, the remaining residue after dissolving in acid being substantially insoluble in ether.

The optimum practical effect is obtained at about 680 C. because, past that point, a copperzinc brass shows dezincification and, in any case, the higher limit of temperature is sharply lim ited by a sintering action then taking place, although some inert filler materials have shown tendencies to delay sintering. Copper powder could be heated successfully to higher temperatures, with increased tarnish resistance, or until sintering takes place.

process can be performed very easily with simple apparatus, various modifications of which are feasible. A possible form giving quite satisfactory results consists essentially of an iron container adapted to be closed capable of holding about 20 pounds of bronze powder. Such a cylindrical container having a diameter of 8 inches and a length of 18 inches is suitable.

A gas inlet and gas outlet are formed at the top of the container for circulating therein the gas used as the processing atmosphere. Heating means of any suitable kind may be provided for raising the temperature of the container and contents to a maximum of about '750" C. or so.

As a specific example, the container may be loaded with 20 pounds of a commercial bronze po der which, upon previous analysis is known to contain about 0.5% of a fatty lubricant. Admitting propane gas of the kind sold as fuel (bottled gas), for a time sufficient to displace the air occluded in the powder, the container is heated gradually from room temperature to about 500 C., keeping the gas flowing continuously, through the said container, the heating at high temperature to take about an hour. Thereafter, the temperature of 59* C. is maintained for from 30 minutes to one hour and, then the heating discontinued. The apparatus is allowed to cool to about 40 0., always under the ing atmosphere, opened and the corn i moved.

As a variation, the container may be a rotatable cylinder, revolving around it longitudinal axis and the gas admitted and removed through the supporting axes which are hollow. The tumbling action so imparted to the powder reduces slightly the total treatment time should this reduction prove to be a desirable co! tion.

The powder so obtained exhibits remaf properties best revealed by means of the fOllCiiV" ing aging tests:

1. Paper strips coated with a standard commercial bronze powder approximating 99% copper and 10% zinc were prepared. Said strips, suspended in a glass jar, having at the bottor 5% aqueous solution of ammonium sulphi were entirely discolored to a purple in two minutes, indicating a thick film of copper sulphide formed on the strips.

2. When the sa: e standard bronze powder was treated according to the invention and this treated powder used for coating iresh test strips, it tool: an exposure in the sulphide glass jar e;- tended to one hour beiore a slight discoloration took place. Consequently, only a much tenuous film of copper sulphide was formed, although ex posure to the tarnishing fumes been 30 times longer than for untreated powder.

Heat tests conducted as follows were also indicative of the resistance to tarnis. to be expected in practice:

1. An ordinary bronze powder heated to C. for seven minutes became completely oxidized;

2. A bronze powder according to the invention, heated to C. for twenty minutes, only showed a very slight change of color after that time, the implication being that complete oxidation would require considerably more than 20 m nutes.

From the results above, it can be said that the resistance to tarnish of the treated powder, compared with an untreated one, is at least very considerable.

The choice of the proper atmosphere is an important consideration for the successful performance of the process: it has been ascertained that generally speaking, any gas is suitable which exerts no decarburizing action on the film being formed. This is meant to indicate that said atmosphere either alone or with vapors formed spontaneously during the heating stage, does not prevent the formation of the protective film, nor react with such film to form volatile products.

More specifically, however, all hydrocarbon gases have been found suitable for the present process, such gases as methane, propane and like being eminently successful. Carbon monoxide gave good results also.

An inert atmosphere such as nitrogen or helium can be used, as Well as vacuum, although it should be noted that vapors of organic matter (the powder lubricant) are always present during the heating step and, consequently, condition the vacuum or inert atmosphere to a reducing one. Concurrently, carbon dioxide is only suitable in presence of a surplus fatty lubricant, or stearic acid which is the usual lubricant used during the commercial beating of bronze powders.

Results with hydrogen are bad, the same holding true with ammonia and sulphur dioxide. It is suspected that hydrogen either forms volatile compounds by re hydrogenization of the residual de-hydrogenized film, or prevents decomposition products from the start. Similar reactions, with consequent formation of volatile organic substances, probably occur with ammonia and sulphur dioxide.

So far, for the formation of the protective film, mention has only been made of the products resulting from the decomposition of the lubricant inherently present in all commercial bronze powders. But, protective-films may be formed also through the agency of additional organic solids and liquids, a great number of which have been tested and produce, at temperatures above 500 C., residues all chemically alike. For example, tar oil, naphthalene, anthracene, butyl stearate, olive oil, sugar, cumaron resins, waxes etc. are materials able to form protective films when decomposed according to the invention.

The above consideration is important because, in certain instances, the residual powder lubricant is not present in quantities sufiicient to produce a film having a thickness meeting the desiderata for adequate protection against tarnish. In such cases it has been found feasible, and advisable, to provide the bronze powder with suitableadditives or, in other words, to fortify the powder; such fortification may mean a simple addition of the conventional lubricant or other organic materials, the object in view being a thicker film after decomposition by heat.

In general, only such organic materials are suitable which, when decomposed in the heat range comprised between 300 to 600 (3., leave a residual film.

As examples of suitable and unsuitable materials, the following experiments are in point:

1. A commercial bronze powder containing about .5% stearic acid and heated for minutes at 500 C. in propane gas yielded, by dissolution in nitric acid, a given amount of residual matter;

2. The same original bronze powder, fortified with .5% of cumaron resin, was similarly heated in propane gas for the same time and dissolved in nitric acid; the residual yield in this case was much higher than in powder 1 above, as might be suspected. Aging tests conducted by heat and in presence of hydrogen sulphide confirmed the fact that the powder containing the highest amount of residuum was also the most tarnishresistant, namely: the powder obtained in Example 2 above.

Conversely, when instead of cumaron resin,

urea was used as a fortifier the eventual residue, after dissolution in nitric acid, was smaller and the aging test quickly determined the lowered resistance of this urea-treated powder against tarnish. Apparently, the effect of urea which includes two amino groups and one carbon atom only per molecule is similar to the effect of ammonia causing, it is believed, volatilization of the film as it formed on the powder particles. Thus, urea may create a decarburizing atmosphere.

0n the other hand, however, stearylamine containing 18 carbon atoms and one amino group per molecule was found to be an excellent fortifler.

Gases are also suitable, in certain cases, for fortifying the ordinary bronze powders, said gases being decomposed by heat to leave on the powder particles a residual film imparting thereto a iven resistance to tarnish. Gases such as methane, ethane or propane do not decompose to a noticeable degree below 600 C. However, when acetylene gas is used (which is known to decompose on a copper surface between 350 C. to 400 C.) the residual film formed thereby on the bronze powder will fortify the film already deposited by other agencies, as described previously. Such fortified bronze powders will then exhibit highly increased tarnish resistance comp-ared to powders not so additionally fortified.

The relation between residual decomposed organic matter, time of heating, temperature and the specific character of the organic matter is complex. Experimental evidence shows that tarnish resistance is in direct ratio with the amount of insoluble residual matter; as said amount increases, the tarnish resistance also increases.

Although the temperature is a variable factor, the best practical temperature for optimum results is about 500 C. The time required to obtain a substantial commercially useful film will vary greatly with the temperature. As a rule, on powders coated with stearic acid, good results are obtainable in 30 minutes at 500 0. whereas, at 400 C. comparable results cannot be reached within several hours. Consequently, the process is more sensitive to temperature than treating time.

A rather surprising result in the present process, and one which is open to much speculation, is the color of the decomposed matter, said color being dark although invisible to the naked eye. As a case in point, powders have been produced by this process in which 5% cumaron resin had been added; after processing no visible trace of film or other matter could be noted.

On the contrary, due to the ever-present organic substances in commercial powders, a reduction of the metal oxides inherent in said powders takes place, whereby the particles of powders treated according to the present process become more lustrous and match, more closely, the metallic sheen of the non-oxidized base metal, or metals, constituting the powders.

Summing up, therefore, the present invention envisages a process for treating commercial copper-base bronze powders, said powders being heated in an enclosure permeated with an atmosphere of reducing character, such as propane, the time of treatment varying from onehalf to an hour at approximately 500 C.

The resulting powder exhibits remarkable resistance to tarnishing and oxidizing agents, the particles of this powder being known to be greasefree, coated with a film of invisible character and greatly impermeable to gases, fumes and other corrosive agents.

From the foregoing, it should be clear that the present invention is a great advance in the art of manufacturing bronze powders for the graphic and plastic trades, decoration and other industries. Aging tests conducted so far have proved conclusively that the bronze powders obtained by the invention should resist the action of tarnishing agents very much longer than ordinary non-treated powders.

As stated previously, ordinary bronze powders cannot be incorporated to certain plastics,

whereas the bronze powders treated as described meet the requirement of the plastic industry for certain specific plastics.

Additionally, the bronze powders of the inven tion are heat resistant, a feature that imply technical applications in future out of the scope of ordinary powders. Furthermore, the new powders are grease free and non-leafing in ordinary paint vehicles, their increased tarnish resistance rend ring possible labels and other printed matter not requiring the usual varnish coating.

As substitutes for gold leaf, on book covers, decals and similar applications, the high refiectivity and prolonged bri htness of the new powders offer great possibilities as an inexpensive alternate pigment.

Finally, as an advantage of no mean magni tude, the powders of the invention are obtained relatively cheaply and at a very small outlay for equipment. Thus, properly produced, the new powders should not be much higher in price than normal non-treated powders.

Obviousy, many modifications of the process are possible are comprised within the spirit of the invention or the scope of the subjoined claims. as well as substitutions of materials as are kl'lUWIl. to be chemical equivalents.

Having thus described this invention, what is claimed is:

l. A process for treating commercial, initially fattwlubricated, lamellar copper-base bronze powder, for rendering said powder resistant to tarnish discoloration, comprising confining such a powder within an enclosure, maintaining in. said enclosure a treating gaseous atmosphere, heating the powder at temperatures above 300 C. and below the sintering temperature of the particular powder being treated, continuing the heating until the initial fatty lubricant has been decomposed into a hard, transparent, in-

visible film covering the individual particles of the powder, and using a treating atmosphere having no decarburizing action on the particle's film, said atmosphere consisting of a gas chosen a ong the hydrocarbon gases, nitrogen, helium, ca con monoxide and also carbon dioxide in presence of organic matter vapors.

2. A process as claimed in claim 1, wherein the treating temperature ranges between 300 C. to 750 C.

3. As an article of manufacture, a copper-base powder of lamellar particles, said particles being free of solvent-soluble organic matter and coated with an inorganic invisible film impermeable to fumes and vapors causing tarnish, when obtained according to the process of claim 1.

i. A tarnish resistant copper-base bronze powder of lamellar structure, free from grease and metal oxides and protected against the action of tarnish-producing fumes by a coating, deposited on the powder lamellae, which is transparent, insoluble in organic solvent and of dense character, said coating being sufficiently thick to isolate the lamellae against tarnish While leaving said lamellae with a color of their constitutive unoxidized metal, when obtained according to the process of claim 2.

JOSEPH HALBERSTADT.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,375,879 Wilrle Apr. 26, 1921 2,528,034 Clayton Oct. 31, 1950 OTHER REFERENCES Article from Metal Cleaning and Finishing, vol. 6, No. 9, September 1934; pages 475 and 476. 

1. A PROCESS FOR TREATING COMMERCIAL, INITIALLY FATTY-LUBRICATED, LAMELLAR COPPER-BASE BRONZE POWDER, FOR RENDERING SAID POWDER RESISTANT TO TARISH DISCOLORATION, COMPRISING CONFINING SUCH A POWDER WITHIN AN ENCLOSURE, MAINTAINING IN SAID ENCLOSURE A TREATING GASEOUS ATMOSPHERE, HEATING THE POWDER AT TEMPERATURE ABOVE 300* C. AND BELOW THE SINTERING TEMPERATURE OF THE PARTICULAR POWDEER BEING TREATED, CONTINUING THE HEATING UNTIL THE INITIAL FATTY LUBRICANT HAS BEEN DECOMPOSED INTO A HARD, TRANSPARENT, INVISIBLE FILM COVERING THE INDIVIDUALS PARTICLES OF THE POWDER, AND USING A TREATING ATMOSPHERE HAVING NO DECARBURIZING ACTION ON THE PARTICLE''S FILM, SAID ATMOSPHERE CONSISTING OF A GAS CHOSEN AMONG THE HYDROCARBON GASES, NITROGEN, HELIUM, CARBON MONOXIDE AND ALSO CARBON DIOXIDE IN PRESENCE OF ORGANIC MATTER VAPORS. 